The invention relates to a method of preventing smoldering decomposition (cigar burning) of multiple-nutrient fertilizers containing ammonium nitrate at elevated temperatures by secondary treatment of the granulated, dried and screened fertilizer with ammonia.
The manufacture, storage and handling of compound fertilizers containing ammonium nitrate demands special safety measures. Unlike pure ammonium nitrate, by reason of their content of inert matter, they are not explosive, but, in certain compositions they are capable of exothermic decomposition under the influence of external heat sources. Thus, a temperature of above 200.degree. C. is sufficient to ignite this kind of fertilizer. An ignition of this kind then propagates as a smoldering decomposition even without the access of oxygen, since the fertilizer itself contains the oxygen necessary for combustion in the form of the nitrate.
It is known from smoldering fires which have occurred in fertilizer warehouses and in ships that compound fertilizers tend to decompose in this manner only if they contain chlorides in addition to ammonium nitrate. This, as a rule, is the case in most NPK fertilizers, which, as is well known, contain potassium chloride as a source of potassium.
Fertilizers containing:
Ammonium nitrate, PA1 (potassium) chloride and PA1 dicalcium phosphate
are particularly endangered by smoldering decomposition. Also, the percentage of inert ingredients should not be too high.
These fertilizers may also contain ammonium phosphate, calcium phosphate, calcium nitrate, trace elements and other additives, as well as, under certain circumstances, the reaction products of the main components, for instance, potassium nitrate and ammonium chloride. In fertilizers containing exclusively water-soluble P.sub.2 O.sub.5, for instance also those produced from super-phosphates or double super-phosphates, with Ca(H.sub.2 PO.sub.4).sub.2 as their main ingredient, instances of smoldering decomposition have not generally been observed. In mixtures, however, which do not contain P.sub.2 O.sub.5, e.g., such as mixtures of ammonium nitrate, potassium chloride and ammonium sulfate, smoldering decomposition has been observed.
This smoldering composition ("cigar burning") is frequently observed in fertilizers of the type 15/15/15 and 13/13/21. The fertilizer of the type 10/10/15 and those which contain still more inert matter (and therefore less of the nutrients) do not seem to exhibit this effect.
In order to prevent this cigar burning, several additives have been proposed, for instance, alkaline earth sulfates or carbonates or alkali metal borates or (poly)phosphates.
As described in German Auslegeschrift No. 2,117,355, prevention of this smoldering decomposition is achieved by adding 3-25% ammonium sulfate to the fertilizer during production, while at the same time, the proportion of water-soluble phosphoric acid is increased to 30-70% of the total phosphate content in order to maintain the predetermined ratio of nutrients.
This step suffices in order to prevent a smoldering decomposition of fertilizer at room temperatures of 20.degree. C. to 30.degree. C., even if it should be heated at one point to the ignition temperature of about 200.degree. C.
Recently, cases of fertilizer fires have become known, where, for instance, a barn caught fire and subsequently the fertilizer stored therein ignited because of the effect of the heat generated by the fire of the barn. In all such cases the fertilizer is first heated to a higher temperature and, as soon as the ignition point has been reached, it begins to smolder.
The usual decomposition tests made with fertilizers at normal temperatures (20.degree. C.) can only be conditionally applied to these circumstances. In tests we have conducted, we found, that fertilizers which were non-flammable at normal temperatures, did indeed decompose and smolder at higher temperatures, for instance after having been pre-heated to 60.degree. C. prior to ignition. This effect was to be expected since the heat of reaction of smoldering in this case is not being used to raise the temperature of the fertilizer, but directly contributes to the acceleration of the decomposition reaction.